A minimum proper weight for a corn to be able to have eggs (with hopefully little to no problems) is 300 grams. You can get a gram weight scale almost anywhere..I even think Wal-Mart sells them..Or you could go to ebay..(which is how I got mine..lol).
A common phrase when breeding corns is the three 3's...300 grams, 3 feet long, 3 years old..but a corn that is under the age or under the length but okay in the weight area usually comes out okay in the end..The weight is pretty much one of the more important factors..
Best of luck to you